Gas turbines are widely used for power generation in commercial operations and propulsion in aviation and marine applications. A typical gas turbine includes a compressor at the front, one or more combustors around the middle, and a turbine at the rear. The compressor and the turbine typically share a common rotor, and each includes multiple stages of airfoils or “blades” attached to the rotor. Rotation of the airfoils in the compressor draws in a working fluid, increases the pressure of the working fluid, and discharges the compressed working fluid to the combustors. The combustors inject fuel into the flow of compressed working fluid and ignite the mixture to produce combustion gases having a high temperature, pressure, and velocity. Expansion of the combustion gases in the turbine causes the airfoils in the turbine to rotate to produce work.
A typical rotor system includes a disk, airfoils attached to the disk, and a shaft to connect the airfoil/disk stages. The outer perimeter of the disk is commonly referred to as a disk rim, and the disk rim includes circumferentially spaced disk lugs or posts. An airfoil attachment at the base of each airfoil fits between adjacent disk lugs to hold the airfoil in place. The airfoil attachment may include a dovetail or other complimentary shape to fit in the space between adjacent disk lugs. In this manner, complimentary surfaces between the disk lugs and the airfoil attachment hold the airfoil in place and prevent circumferential or radial movement of the airfoil during operation. Various techniques are used to provide axial restraint of the airfoil attachment. Industrial gas turbines, for example, utilize “staking” or locally deforming the base of the airfoil attachment. In addition, this mechanism facilitates easy removal and replacement of defective or worn airfoils as the need arises.
During operations, the disk typically rotates at speeds exceeding 3,000 rpm. A typical clearance between the disk lugs and the airfoil attachments is on the order of 0.001 to 0.002 inches to facilitate insertion of the airfoil attachments. In service, the airfoil attachments may move from 0.000 to 0.002 inches with respect to the disk lugs in a cyclic manner due to airfoil vibrations. This small relative motion can result in fretting wear between the airfoil attachments and the disk lugs. After extended periods of operation, the fretting wear may create cracks at the edge of contact locations on the airfoil attachments and/or disk lugs, potentially leading to premature failure and release of the airfoils.
A hard coating may be applied to the surfaces of the airfoil attachments to prevent or reduce the amount of fretting wear on the airfoil attachments. Various methods exist to apply the hard coating to the airfoil attachments before they are installed between the disk lugs on the disk. For example, the hard surface may be applied to the surfaces of the airfoil attachments using a plasma spray gun. This application method and others are relatively easy to accomplish before the airfoils attachments are installed between the disk lugs because of the readily available access and line of sight to the airfoil attachments before installation as required by typical coating deposition processes. However, the surfaces of the disk lugs are not readily accessible, and the space between adjacent disk lugs is typically not sufficient to allow the use of a plasma spray gun or similar applicator to apply the hard coating to the surfaces of the disk lugs.
Therefore, the need exists for an improved system and method for installing a hardened coating on the surfaces of disk lugs. Ideally, the improved system and method may allow for the installation of hardened coatings on the surfaces of the disk lugs, and the hardened coatings may be readily replaced after regular periods operations or whenever the need arises.